Datasheet

MCP16301
DS25004A-page 18 © 2011 Microchip Technology Inc.
5.4 Input Capacitor Selection
The step-down converter input capacitor must filter the
high input ripple current, as a result of pulsing or
chopping the input voltage. The MCP16301 input
voltage pin is used to supply voltage for the power train
and as a source for internal bias. A low equivalent
series resistance (ESR), preferably a ceramic
capacitor, is recommended. The necessary
capacitance is dependent upon the maximum load
current and source impedance. Three capacitor
parameters to keep in mind are the voltage rating,
equivalent series resistance and the temperature
rating. For wide temperature range applications, a
multi-layer X7R dielectric is recommended, while for
applications with limited temperature range, a multi-
layer X5R dielectric is acceptable. Typically, input
capacitance between 4.7
µF and 10 µF is sufficient for
most applications. For applications with 100
mA to
200
mA load, a 1 µF X7R capacitor can be used,
depending on the input source and its impedance.
The input capacitor voltage rating should be a minimum
of V
IN
plus margin. Table 5-2 contains the
recommended range for the input capacitor value.
5.5 Output Capacitor Selection
The output capacitor helps in providing a stable output
voltage during sudden load transients, and reduces the
output voltage ripple. As with the input capacitor, X5R
and X7R ceramic capacitors are well suited for this
application.
The MCP16301 is internally compensated, so the
output capacitance range is limited. See
Table 5-2 for
the recommended output capacitor range.
The amount and type of output capacitance and equiv-
alent series resistance will have a significant effect on
the output ripple voltage and system stability. The
range of the output capacitance is limited due to the
integrated compensation of the MCP16301.
The output voltage capacitor voltage rating should be a
minimum of V
OUT
, plus margin.
Table 5-2 contains the recommended range for the
input and output capacitor value:
5.6 Inductor Selection
The MCP16301 is designed to be used with small sur-
face mount inductors. Several specifications should be
considered prior to selecting an inductor. To optimize
system performance, the inductance value is deter
-
mined by the output voltage (Tabl e 5-1) so the inductor
ripple current is somewhat constant over the output
voltage range.
EQUATION 5-4: INDUCTOR RIPPLE
CURRENT
EXAMPLE 5-3:
EQUATION 5-5: INDUCTOR PEAK
CURRENT
An inductor saturation rating minimum of 760 mA is
recommended. Low ESR inductors result in higher
system efficiency. A trade-off between size, cost and
efficiency is made to achieve the desired results.
TABLE 5-2: CAPACITOR VALUE RANGE
Parameter Min Max
C
IN
2.2 µF none
C
OUT
20 µF none
Δ
I
L
V
L
L
------t
ON
× =
V
IN
=12V
V
OUT
=3.3V
I
OUT
=600 mA
I
LPK
Δ I
L
2
-------- I
OUT
+=
Inductor ripple current = 319 mA
Inductor peak current = 760 mA